tl;dr: If what led you here was a search as to whether Unity3D can be used as a tool to create web games (as of November 2015)... the answer is:

Nope. Not yet. Sorry.

The longer story:

Gaming on the web used to be ginormous. Companies like Kixeye, Zynga, Playdom, Digital Chocolate, and Kabam got fat and happy making quick-to-consume, easy-to-spread games that ran right on Facebook.

What made these games viable was Adobe's Flash -- a flawed but outstanding toolset to build, playtest, and deploy web games. Artists, animators, and engineers all came to know and love (well, tolerate) Flash.

To a lesser extent, some games were also released via Unity3D's web player, which was a plug-in allowing games -- even harder-core 3D content -- to run fairly well right in the browser.

Then Facebook gaming became saturated. Surging viral channels became parched riverbeds and it became expensive to acquire users.

But as Facebook flailed, mobile flourished. Unity3D was poised to become a juggernaut -- a spectacular set of tools that took away tons of the tedium and pain of game-making and played a huge role in powering the proliferation and quality of mobile games. The same code-base, with a few button presses, could be deployed to iOS, Android, PC, Consoles, as as well as the browser!

Meanwhile, browser-makers began realizing they had a security risk on their hands. Having a plug-in with the ability to run arbitrary code meant that all sorts of viruses and malware could sneak in. This was a different kind of viral channel -- not one you wanted rampant on your PC.

So here we sit, in the nether regions of 2015, with browsers (wisely) dropping support for the Flash Player, Unity Web Player, and other plug-ins.

And there seems to be nobody in the web game business with the clout to do anything about it.

So, with a whimper, we must first accept the fact that Flash and the Web Player are dying. As long-time makers of web games, our company is still a bit in denial.

Are browser games dead?

Playing games right in the browser still has tons of awesomeness -- for everyone from the bored office drone sneaking a Farmville harvest in one of her tabs to the kid snacking on mini games on Kong or Addictinggames between classes to the power-user who wants to keep villages producing gold while keeping an eye out for enemy attacks. Gaming is ubiquitous and easy!

And for the publisher, web games are way easier to share, promote across any device, and far cheaper to target and acquire the right customers than packed mobile app stores.

So what's a fan of web gaming to do?

Many blogs and mailing lists tell us not to worry. WebGL will save the day -- in particular Unity3D's ability to spit out working WebGL. WebGL is an ultra-cool standard for graphics that runs natively right inside the browser.

But this is why it's not yet viable:

Unity3D's magic works by taking C# code, compiling it (IL code), using the IL2CPP converter to create C++, then translating that to JavaScript/WebGL.

It's pretty amazing acrobatics, but it means:

Debugging hell -- basically stuck using logging to a console

Highly limited code accessibility and readability

HUGE download size (300MB just for the Unity library -- never mind the game)

Difficult to compress, stream, or otherwise optimize

Add to that, that the whole effort is kinky (not in a good way):

Unity3D's latest version has known and unknown bugs, and tons of features that (understandably) aren't on parity with WebGL's standards

The IL2CPP toolchain has continual issues
Browsers each have their different WebGL problems or interpretations

(a common experience)

But the news isn't all doom and gloom.

Smart people ARE working hard on all of the above kinks, trying to smooth things out.

New browsers will have better and more unified WebGL support.

Unity3D seems like it will continue investing in figuring out the technology.

But in the meantime, we believe the only viable option for web games today is HTML5.

Luckily, many great games can be built in HTML5 (see some of our own for examples). Though development is way-way more complex than it would be with a tool like Unity.

We've fallen so far so fast from the golden age of web gaming. But give it a few cycles and, there is little doubt: A mighty new era of browser games shall be unleashed upon us.